The present invention relates to a method as well as to a device for attenuating the motion of hydraulic cylinders of mobile work machinery, in particular of hydraulic excavators, wherein, by means of a position registering device, reaching of a preliminary limit position of the hydraulic cylinder is registered; prior to the limits of travel of the hydraulic cylinder being reached, its motion speed is reduced; and the hydraulic cylinder is moved to the respective limit of travel only at reduced speed. To this effect a flow control device for throttling the inflow to, and/or the outflow from, the hydraulic cylinder is provided, with said throttling being correspondingly driven by a control device when the preliminary limit position is reached, so as to throttle the flow quantity which flows into, or out of, said hydraulic cylinder.
The motion attenuation or limit of travel switch-off of hydraulic cylinders ensures that the speed of the hydraulic cylinders is reduced shortly before the mechanical limit stop is reached, in order to prevent excessive mechanical loads acting on the steel components, due to inertia forces resulting from the abrupt delay, and in order to increase the level of work comfort. Hydraulic solutions as well as electrical switch-off devices have already been proposed for such motion attenuation.
FIG. 7 shows a hydraulic solution. As shown in said Figure, hydraulic cylinders of earth-moving machinery such as hydraulic excavators and the like are regularly driven by way of a hydraulic pump 1 and a directional control valve 4 arranged downstream of said hydraulic pump 1. In the limit region of the piston and the rod, the hydraulic cylinder 10 comprises geometric changes 13 which cause a pressure buildup of the returning fluid at the time of entry into the changed geometry 12 of the cylinder housing.
The speed of the cylinder is determined by way of the flow rate of the hydraulic pump 1 in the inflow to the cylinder. An attenuation effect is generated only if the quantity in the inflow to the cylinder is reduced. In this arrangement, a reduction can only be achieved in that either the regulator R of the pump 1 or a pressure relief valve 7, which forms part of the hydraulic circuit, responds. In this arrangement, a response of the pump regulator or of the pressure relief valve is achieved by the inflow pressure, which means that the banking-up pressure on the outflow side has to increase in line with the transmission ratio of the hydraulic cylinder. Depending on the size of the machine, the pressure regulator of the pump, or the pressure relief valve respectively, responds at between 300 and 350 bar pressure, so that a banking-up pressure of 600 to 700 bar is required on the inflow side of the hydraulic cylinder.
The banking-up pressure is achieved via throttling at the annular clearance and via special throttle cross-sections, wherein the throttle effect at the annular clearance greatly depends on manufacturing tolerances and the viscosity of the fluid. Due to these deviations of parameters relating to the geometry and the fluid, there is a good likelihood that either the banking-up pressure is insufficient to activate the control devices, or that the banking-up pressure increases to such an extent that the integrity of the cylinder housing is endangered.
Due to these shortcomings, electrical switching off of the inflow and outflow has been proposed. In systems with electro-hydraulic pilot control, electrical switching off has been used in which one limit switch is provided for each movement direction of the cylinder. Shortly before the cylinder reaches its limit of travel, a respective limit switch is overtravelled, with the signal of said limit switch prompting the control device to switch the respective directional control valve off. This results in the motion being decelerated, depending on the switching speed of the directional control valve.
However, with this solution, stopping regularly takes place either too early or too late. This means that either the kinematics are not completely utilised, or that the mechanical limit stop of the hydraulic cylinder is still reached at excessive speed. Furthermore, during uncontrolled switching off, pressure peaks occur on the outflow side, while the inflow side is filled incompletely, with both of these occurrences leading to increased loads on the lines and hydraulic components.